1. Field of the Invention
The present invention relates to an apparatus for measuring an air flow rate by using a heat generating resistor, and more particularly to an air flow rate measuring apparatus advantageously utilized for controlling the air-fuel ratio of an internal combustion engine of an vehicle.
2. Description of the Related Art
It has been well known that one of apparatus for measuring a flow rate of a fluid such as air is a flow rate measuring apparatus of a heat generating resistor type.
The flow rate measuring apparatus of the heat generating resistor type is utilized in such a manner that the heat generating resistor supplied with energy electically heated is provided in a fluid of which flow rate is under measurement. Then, the flowing velocity is detected by using that the quantity of heat taken away from the heat generating resistor by the fluid becomes a function of the flowing velocity, whereby the flow rate is measured.
Recently, because of the advantages that will be described later on, the flow rate measuring apparatus of the heat generating resistor type has been widely utilized for measuring an intake air flow rate for the air-fuel ratio control of an engine (internal combustion engine) of a vehicle.
According to the flow rate measuring apparatus of the heat generating resistor type, it is possible to measure the mass flow rate directly. For this reason, there can be obtained advantages that data obtained therefrom need not be corrected based on the atmospheric pressure or temperature.
Further, the flow rate measuring apparatus of the heat generating resistor type has a nonlinear characteristic such that the relationship between the flow rate and the detected signal exhibits a steep slope in the low flow rate region and the slope becomes gentler as the flow rate is increased. For this reason, it is possible to obtain a wide measurement range in which an error rate need not be differently set. Moreover, even if the flow rate is small with the result that the flow rate exhibits only a small change, it is possible to obtain an output voltage change large enough to be detected by an analog-to-digital converter with a reasonable resolution. Accordingly, the flow rate measuring apparatus of the heat generating resistor type is extremely advantageous in application to a control of idling engine speed for stabilizing the same.
Conversely, due to the nonlinear characteristic, the flow rate measuring apparatus requires a correction processing for linearizing the output. For this reason, in a prior art, as for example shown in FIG. 11, a flow rate signal V generated from a flow rate detecting unit 3 is supplied to an engine control unit 2 in which the flow rate signal V is subjected to a linearizing processing and also subjected to an averaging processing by a filter upon necessity. Thus, data indicative of an air flow rate Q is obtained.
The engine control unit 2 calculates a fuel injection amount for an engine under consideration of other parameters such as a throttle valve opening degree, an engine speed as shown in the figure. Thus, the air-fuel ratio for the engine is controlled for the engine not shown.
The flow rate detecting unit 3 is composed of a heat generating resistor 3a as a detecting element. The heat generating resistor 3a is provided within an air passage A such as an intake manifold of the engine or the like so that the heat generating resistor 3a is exposed to an intake air flow AF.
On the other hand, there has been proposed a prior art arrangement of the flow rate measuring apparatus of the heat generating resistor type in which, as shown in FIG. 12, influence of backward flow caused from an intake air ripple of the engine within the intake manifold is corrected.
According to the prior art arrangement, as shown in FIG. 12, a flow rate detecting unit 30 is provided with, in addition to the original heat generating resistor 3a, another heat generating resistor 3b serving for detecting the backward flow of intake air with this arrangement, data indicative of an air flow rate Q having been subjected to the backward flow correction can be obtained.
A prior art relating to this kind of correction can be found in Japanese Patent Laid-Open No. Hei 8-94406, for example.
The above-described prior art, however, does not take into account deterioration in detection precision due to the averaging processing which is carried out by a filter in addition to the linearizing processing on the flow rate signal. Thus, the prior art will encounter the following difficulties.
As described above, the flow rate measuring apparatus of the heat generating resistor type has some advantages and disadvantages.
One of advantages is that the mass flow amount can be directly measured, the data obtained therefrom need not be corrected based on the change in atmospheric pressure or temperature. Another advantage is that the flow rate measuring apparatus of the heat generating resistor type has a nonlinear characteristic that the relationship between the flow rate and the output voltage exhibits a steep slope in a low flow rate region while a gentle slope in a high flow rate region. Therefore, even when the flow rate is low and thus it exhibits small flow rate fluctuation such as when the engine is placed in an idling drive mode, it is possible to obtain an output voltage fluctuation amount large enough to be detected by an analog-to-digital converter with a reasonable resolution, which fact is useful for stabilizing the idling speed of the engine.
Conversely, the flow rate measuring apparatus of the heat generating resistor type has a drawback that the nonlinear characteristic causes a measurement error.
This phenomenon is caused because the relationship between the flow rate and the output voltage is not linear, with the result that the mean value of the output voltage is decreased with respect to the mean value of the flow rate due to the engine speed ripple or the like.
This phenomenon particularly acts on increase in the air-fuel ratio (the ratio of fuel to air is decreased) in view of the engine control standpoint, leading to decrease in output of the engine.
Now, the drawback of the prior art will hereinafter be described.
FIGS. 13 and 14 show the relationship between the ripple amplitude and the detected voltage of the heat generating resistor of the flow rate measuring apparatus of the heat generating resistor type according to the prior art arrangement shown in FIG. 11.
These diagrams are characteristic diagrams in which the air flow rate Q is plotted in abscissa while the voltage value V of the detected signal detected by the heat generating resistor 3a is plotted in ordinate. In this case, FIG. 13 is a characteristic diagram in which the engine control unit does not carry out the averaging processing with a hard filter while FIG. 14 is a characteristic diagram in which the engine control unit carries out the averaging processing with a hard filter or the like.
Since air flowing the intake manifold of the engine ripples with the opening and closing motion of the intake valve, the detected air flow rate is also rippled as shown in the figures.
Since the temperature of the heat generating resistor 3a substantially faithfully responds to the fluctuation of the air flow rate, the detected signal also ripples as shown in the figures.
At this time, since the voltage value characteristic of the detected signal detected by the heat generating resistor 3a relative to the air flow rate exhibits nonlinearity as shown in the figure, the mean value of the detected signal is decreased relative to the mean value of the original air flow rate Q, which fact leads to a measurement error derived from the nonlinearity of the heat generating resistor and the rippled amplitude.
However, if the heat generating resistor could detect the intake air ripple without any response delay, the detected ripple could be sequentially converted into the air flow rate, and the mean value thereof could be calculated, then the decrease in the means value as describe above should be avoided regardless of the deviation in the mean value of the detected signal.
However, even if the intake air ripple can be detected without any response delay by the heat generating resistor, it is very difficult to convert the intake air ripple component into the air flow rate continuously. Therefore, it is almost impossible in view of practical standpoint, since the air flow measuring apparatus of the heat generating resistor type is installed in an engine room of a vehicle, and the air flow rate measuring apparatus suffers from ignition noise or intake air disturbance which cause error in detected data. Further, too much calculation task can be imposed on a signal processing unit of the engine control unit.
For this reason, the prior art arrangement of the air flow rate measuring apparatus of the heat generating resistor type employs a hard filter for averaging the output voltage when the output voltage is supplied to an analog-to-digital converter in the engine control unit 2. A filter selected as the hard filter is ordinarily one composed of a resistor and a capacitor having a time constant of 1 to 20 ms.
As set forth above, FIG. 14 shows a characteristic of the air flow rate measuring apparatus when the apparatus employs a hard filter. Since the hard filter acts on the characteristic such that the amplitude is made small without changing the mean value of the output voltage value. Therefore, if the detected value is directly converted into the air flow rate, an error that the mean value of the converted air flow rate is also decreased is caused, resulting in a characteristic different from that shown in FIG. 13. In this case, however fast the analog-to-digital converter carries out the sampling operation, error will be caused in the detected flow rate value.
Recently, there are many cases in which two heat generating resistors are employed to detect the backward flow of air generated in the intake manifold and data is corrected based on the detected backward flow amount, whereby detecting precision of the flow rate measuring apparatus is increased. In this case, however, small backward flow will cause a large fluctuation in the output voltage. Therefore, averaging operation on the value of the output voltage gives an excessive backward flow amount compensation.
The above-mentioned drawback can be avoided by carrying out data sampling at a high rate and linearizing processing continuously. However, in order to avoid the drawback, it is necessary to carry out the processing at a sampling rate of at least 1 ms. Therefore, the engine control unit will be loaded with a heavy duty such as a high sampling speed, a large arithmetic operation task or the like that will not be requested when other type of sensors are employed.
In detail, the prior art arrangement shown in FIG. 12 distinguishes the direction of air flow in such a manner that when the detected output voltage V is larger than the threshold value Vm (air flow ratio=0), the direction of the air flow is determined forward while when the detected output voltage V is smaller than the threshold value of Vm, the direction of the air flow is determined backward. In this case, similarly to the prior art shown in FIG. 11, if the prior art arrangement converts the output value into the air flow rate continuously without using a hard filter, the characteristic thereof becomes as shown in FIG. 15. At this time, as far as the mean value of the output voltage is concerned, measurement error excessive with respect to that in the case of FIG. 12 is caused. However, after the output voltage is converted into the air flow rate continuously, measurement value does not contain error.
However, if the output voltage is converted into the air flow rate by way of a hard filter, as shown in FIG. 16, measurement error is left unremoved and it becomes impossible to detect the backward flow. Therefore, the original purpose, i.e., to detect backward flow cannot be accomplished.
As described above, the air flow rate measuring apparatus of the heat generating resistor type provides not only advantages but also disadvantages due to its nonlinear. characteristic, and hence it is difficult to take only an advantage thereof at the current stage, which fact is recognized as a problem to be solved.
Therefore, an object of the present invention is to provide an air flow rate measuring apparatus of a heat generating resistor type in which it is possible to adequately take the advantage of the air flow rate measuring apparatus of a heat generating resistor type and to obtain a high control precision when the air flow rate measuring apparatus of the heat generating resistor type is applied to the engine control.
In order to attain the above object, there is provided an air flow rate measuring apparatus having a flow rate detecting unit which is utilized for measuring air flow rate using a heat generating resistor provided in an air flow passage, including a linearizing circuit inputted with a signal indicative of a flow rate having a nonlinear characteristic from the flow rate detecting unit, a filter circuit inputted with a signal supplied from the linearizing circuit, and a nonlinear-form converting circuit inputted with a signal supplied from the filter circuit, wherein the output of the nonlinear-form converting circuit is extracted as a flow rate detecting signal.
Further objects and advantages of the present invention will be apparent from the following description which is given with reference to the accompanying drawings wherein preferred embodiments of the present invention is clearly shown.